The present invention relates to a compound semiconductor integrated circuit using superspeed field effect transistors (FETs) and an optical regenerative repeater using such a circuit and more particularly to a compound semiconductor integrated circuit having an element separating structure which is suited to increase operation speed.
The distance between elements forming an integrated circuit has been reduced as the high integration of semiconductor integrated circuits has been recently improved. With the reduction of the inter-element distance, a problem which is called a sidegate effect is imposed. The sidegate effect is a phenomenon that a current flowing through a field effect transistor (FET) is decreased or the threshold voltage is changed by the electric interaction between the FET and other elements.
When creating an amplifier circuit with a high gain using FETs having compound semiconductors, it is important to control the sidegate effect as well as a phenomenon which is called low frequency oscillation. The phenomenon of low frequency oscillation is a phenomenon in which a current flowing through an FET in an integrated circuit self-oscillates stationarily even if no signal is inputted and is called "low frequency oscillation" because the oscillation frequency is very low, such as several Hz at room temperature.
A conventional compound semiconductor integrated circuit using FETs is indicated, for example, in Japanese Patent Application Laid-Open No. 2-49465. As shown in FIG. 3 of this application, the compound semiconductor integrated circuit has element separating zones 39, which reach a semi-insulating GaAs substrate 31, between neighboring FETs (T1, T2, T3) so as to improve the electric separation between the FETs, particularly the sidegate effect. In FIG. 3, symbols T1, T2, and T3 indicate FETs which are called HEMT, numeral 32 an undoped GaAs buffer layer, 33 an n type AlGaAs electron supply layer, 34 an n type GaAs contact layer, 35 an ohmic electrode, 37 a gate electrode, and 38 an element separating zone which does not reach the substrate 31.
Another conventional example is indicated, for example, in Japanese Patent Application Laid-Open No. 3-87044. As shown in FIG. 4 of this application, the compound semiconductor integrated circuit has a hetero-junction buffer layer 42 with a thickness of 1000 .ANG., which is made of AlGaAs, under the GaAs FETs and element separating trenches 46, which reach a hetero junction interface 44, between the neighboring FETs so as to improve the electric separation between the FETs, particularly the sidegate effect. In FIG. 4, numeral 41 indicates a semi-insulating GaAs substrate, 43 a GaAs layer, 45 an ohmic electrode, 47 and 48 ohmic electrodes, and 49 a gate electrode. The element separating trenches 46 may be filled with an insulator so as to form material separating adjacent elements. In this case, due to the element separating trenches 46 or furthermore the insulator with which the trenches are filled, it is necessary that a leakage current between the elements would have to pass through the hetero-junction interface 44. However, the leakage current is prevented by the energy barrier of the hetero junction interface 44 and does not flow into the neighboring elements.
According to the above prior art, the control effect for low frequency oscillation is imperfect and an integrated circuit by the prior art performs a faulty operation. For example, when a limit amplifier with a gain of 50 dB is created, low frequency oscillation generated in the integrated circuit is amplified and low frequency oscillation noise which is large enough to saturate the output amplitude often occurs.
According to the prior art, the element separating trenches are formed by the dry etching method, so that the sides of the element separating trenches are almost perpendicular to the surface of the semiconductor integrated circuit. Therefore, the insulator surface in the neighborhood of each side of the element separating trenchers which are filled with an insulator is not sufficiently flat (a level difference is generated) and the wiring metal deposited on the insulator surface is not uniform in thickness, for example, the wiring metal formed in each trench is thick. As a result, in the wiring metal etching process for forming wires, an etching residue is generated in each trench and wires are connected, that is, short-circuited. As a method for solving this problem, a method for forming element separating etched regions by the wet etching method instead of the dry etching method is possible. It is known that by the wet etching method, each side that is a part of a material to be etched which is removed by etching is formed as a slope and the slope can be processed in an inclined state toward the center of the part to be removed by etching in the depth direction from the surface. Therefore, when an element separating etched region with such a shape is filled with an insulator, the insulator surface in the neighborhood of the etched region is flat and the wiring metal deposited on the insulator surface is uniform in thickness. As a result, the problem of short-circuiting of wires can be solved. However, when an etched region is formed by the wet etching method, a new problem is imposed in that the integration density is reduced by the area required for the side slope.